End plate potentials (e.p.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce tansmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. The authors will refer to this process as augmentation. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 sec (range 4-10 sec) which is intermediate in duration between the time constants for the decay of facilitation and potentiation. The magnitude of augmentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0.3 to 0.6 following 50 impulses at 20/sec to 0.5-7.8 following 600 impulses at 20/sec. (An augmentation of 0.3 and 7.8 would increase e.p.p. amplitudes 1.3 and 8.8 times, respectively). The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3.8. The corresponding Q10 for the decay of potentiation was found to be about 2.4. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.
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